The present invention relates generally to data-recording media, i.e., records, and to record track-following and identifying techniques. More particularly, it relates to improved track-identification patterns and to improved systems for utilizing such patterns, both in track-identification and track-following, and in the recording of data in designated areas along such tracks.
More particularly still, in its most preferred form the invention relates to recording tape for use in data-recording systems, having pre-recorded track-identifying and track-following servo information recorded thereon, together with the preferred systems for utilizing such pre-recorded tape-form record members. In broader aspects of the invention, however, the encoding and decoding techniques for track identification, and the improved record format involved, are also applicable to other forms of record member or media, including discs.
In the use and continued development of data recorders, particularly magnetic recorders, substantial effort and progress continues toward the goal of increasing the amount of data for a unit amount of surface area on the recording media. This involves not only increased bit-packing densities, but also involves substantial increases in the number of data tracks per given area, i.e., the tracks themselves being narrowed, and also being placed closer together.
In the past, the higher range of track densities have been found primarily in the use area of disc-type recorders, first in large-capacity rigid or "hard" discs, and more recently in some flexible disc applications. While magnetic recording tape has long been used as a data storage medium, a high density multiple-track data storage tape is rarely or never encountered, particularly in business office-type user applications, even though possessing rather substantial potential advantages and benefits. The present invention provides such multiple-track recording tape, together with systems and methods for utilizing the same, in which the multiple data tracks are characterized by the presence of "embedded servo" tracking information, preferably in dedicated areas located near the ends of the tape, as well as at locations disposed along the length of the tape, interspersed with or disposed between data-recording regions.
In achieving high track densities in the use of disc-form media, it has heretofore become known that the recording tracks were rarely perfectly concentric with one another and circular in shape; consequently, when the tracks are very narrow and very closely spaced to one another it becomes necessary to servo-position the transducer, and to employ closed-loop techniques, so that the transducer follows the particular motion of the disc, actually following the motion of the tracks recorded on the disc.
In the case of recording tape, the conditions incident to high track densities and lateral tape "wander," as well as other irregularities of motion or non-stabilities, have not heretofore been substantially appreciated or well-understood. It is found, however, that recorded tape exhibits its own pecularities of motion as it is transported lengthwise past a transducer, as a result of the seeming impossibility of guiding the tape so continuously and so completely as to maintain this motion in an essentially perfect laterally-fixed position. Of course, recording tape is typically under tension as it is transported lengthwise, and the tape itself is subject to a certain amount of stretching, the amount of which varies as a result of the wide variations in drive forces applied during these conditions. Further, lateral excursions of comparatively large magnitude both parallel to and perpendicular to the transducer may also occur, especially during stopping and starting conditions, as well as during steady-state longitudinal transport. Because of the somewhat random occurrence and presence of such conditions, the non-uniformity of lateral movements of the tape itself, and of recording tracks on the tape, becomes an increasingly important factor as track density increases. Thus, with increased track density, it becomes desirable, and indeed necessary, to servo-control the transducer, moving the same laterally with respect to the tape as the tape is transported longitudinally adjacent the transducer, such that the transducer in effect follows the tracks recorded on the tape even though they may move in a more or less continuous manner with respect to a fixed reference as the tape passes the transducer, or head.
Thus, while there are certainly some general similarities or analogous points involved in magnetic recording on discs as compared to tapes, there are fundamental differences which to a considerable degree isolate the two different activities from one another. That is not only true with respect to the types and natures of the inaccuracies or irregularities of motion, but is also true with respect to conditions for servoing the respective recording media. For example, in the case of disc recorders, it has long been an accepted practice to use at least one track, and frequently either additional tracks or one entire side of a disc (in a multi-disc environment) exclusively for permanently-recorded clock and servoing information or signals. In such situations, a completely separate transducer head is also utilized, being dedicated to clocking and/or servoing activities, and not used for other purposes such as data recording or replay. Since most recording tape is relatively narrow, where multi-track arrangements are desired it is antithetical, and undesirable, to dedicate an entire track or area for permanent use as a source of servo or clock signals. Of course, the provision of a dedicated transducer head for such purposes is also undesirable from the standpoint of economics, at least, and the use of such a head also may present strictures or difficulties from a packaging or placement standpoint, particularly in the case of tape, which is narrow in width and which has most of its length in an unaccessible position, i.e., wound upon reels of one type or another. Consequently, other approaches become highly desirable, if not mandatory, consistent with the particular conditions presented by the nature of recording tape and its environment.
In considering the general development of record track-identification and following systems, generally as an adjunct of high track densities and narrow track widths, prior developments which have taken place in disc-recording technology are to some extent helpful; however, as pointed out above, the inherent nature of tape-recording techniques, of the tape media itself, and of particular conditions mandated thereby, combine to present their own particular problems. For example, recording tape often commences motion from a rest position, which may either be at one end of the tape or somewhere between the ends thereof; furthermore, tape drive motion may in many instances be bi-directional, or various bands or tracks on the tape may be intended for a first direction of motion, whereas other bands or tracks are intended for the opposite direction of motion. Thus, "embedded servo" techniques developed for use in disc applications may well not be applicable, or may raise quite different problems if applied.